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Cannabis and mental health

All rapid responses

Rapid responses are electronic letters to the editor. They enable our users to debate issues raised in articles published on thebmj.com. Although a selection of rapid responses will be included online and in print as readers' letters, their first appearance online means that they are published articles. If you need the url (web address) of an individual response, perhaps for citation purposes, simply click on the response headline and copy the url from the browser window. Letters are indexed in PubMed.

My BMJ failed to arrive so I missed the contributions on cannabis
until I got onto the BMJ web-site today. A disease can be fully
defined by the 1. the causal agent,2 the lesion and 3. the syndrome. In
the case of cannabis we are not clear about the lesion but the syndrome
associated with cannabis is here called schizophrenia. Logically then we
should refer to cannabis schizophrenia. The incidence and prevalence of
patients showing schizophrenic syndromes are unchanged or have even fallen
while the use of cannabis has increased enormously. We must conclude that
either previous schizophrenic illnesses have become much less common or
that cannabis schizophrenia is rare and perhaps it may not even exist.

Rey and Tennant (1) discuss the strong evidence linking cannabis use
(especially heavy cannabis use) with psychosis. They speculate on the
importance of different developmental stages but do not elaborate on this
subject. It is possible that such a process explains two recent findings:
cannabis use at age 15 was more strongly associated with psychosis at age
26 than was cannabis use at age 18 (2); prisoners in England and Wales
were more likely to have a functional psychosis if they had started using
illicit drugs, including cannabis, before age 16 (3).

This suggests that there may be a period of developmental
vulnerability during which initiation of cannabis is associated with an
increased rate of developing schizophrenia or a functional psychosis.
However the vast majority of cannabis initiation occurs over a very short
period of time. The risk of first use rapidly increases from age 11 and
then rapidly decreases from age 18 until by the mid-20s the risk of
initiation is minimal (4).

If a period of developmental vulnerability does exist in early
adolescence then it may be that later initiation of drugs (i.e. from age
18 +) is less likely to result in heavy or dependent use and may not be
associated with the development of a psychosis. If this were the case it
might explain the finding that Swedish conscripts with the lowest use of
cannabis had the same odds of developing schizophrenia when compared with
conscripts who reported no use of cannabis (5).

This is an extract of a letter I sent to David Blunket 7th Nov 2001
(yes a year ago):

Schizophrenia is the single most costly illness in the NHS budget

The Cannabis of today is up to 30x more powerful than the drug used
by some of our legislators during their student days 20-30 years ago. To
illustrate this clearly the dosage increase is the same proportion as
changing one’s alcohol consumption from a glass of wine of 1½ bottles of
Whisky. The effect and consequences of the different doses are far apart

For relief to the symptoms of some medical conditions it is clear
that Cannabis has benefit

The issue of decriminalising Cannabis to a category C drug is the
worst situation for this drug. It should either continue to be a category
B drug (unworkable) or legalised with appropriate warning to users along
the lines used on tobacco, for example:
· ‘For people susceptible to Schizophrenia Cannabis can trigger the
disease’
· The THC content of the Cannabis should also be indicated along the lines
of the tar information on cigarettes today

Decriminalising Cannabis may lead to increased consumption and
therefore could lead to an increase in demand for treatment for
schizophrenia and hence further pressure on the NHS budget

Competing interests:
None declared

Competing interests:
No competing interests

01 December 2002

Hugh N Roberts

Parent of a patient

Not Applicable. I'm employed in the Advanced Materials Technology Industry

While the BMJ editorial on the whole analyses the data well, Two most
glaringly obvious observations from the original Swedish conscript study
have been overlooked. That is that most of the people taking cannabis did
not develop psychosis, and conversely most people who developed
schizophrenia (197/41280) did not use cannabis. While the incidence in
cannabis users increases, and there is a dose-response relationship (the
higher the dose the greater the chance) the fact remains that out of those
2836 using 1-10 occasions only 18 developed schizophrenia (RR 1.3) 10/702
using cannabis 11-50 times developed schizophrenia (RR 3.0), and 21/752
using >50 times (RR 6.0).

Clearly there is a seed-soil phenomenon and cannabis may contribute
to the web of causation in some susceptible people, but cannabis by itself
is not the cause of schizophrenia.

The possible association between cannabis and severe mental illness,
such as schizophrenia, goes round in circles. A correspondence in the BMJ
(Vol 2 1893, pp. 630, 710, 813-4, 868-9, & 969, variously entitled
'Insanity from the abuse of Indian hemp', 'Indian hemp as a cause of
insanity')conducts just the same arguments as outlined by Rey and Tennant
(BMJ 2002: 325, 1183-4). The robust 1893 BMJ correspondent Pyramid stated
clearly that "the employment of hashish in Egypt has a beneficial effect
on the almost vegetarian population". By contrast Dr Thomas Ireland,
Government Medical Officer in British Guiana, considered that “the
excessive use of Indian hemp […was] a very prolific cause of insanity”.

The fact is that increased usage in the western world has not led to
any increase in the incidence of schizophrenia (if anything it is
declining), nor shown any geographically located increase. In over 20
years of inner city psychiatry I have never seen a case of ‘cannabis
psychosis’, and there is no evidence of a specific or characteristic
psychopathology suggesting that this is a diagnostic entity (1). By
contrast, schizophrenic patients questioned about cannabis usage by our
staff consistently state that they find this drug makes them feel ‘good’
or ‘calmer’, while those feeling uncomfortable, ‘bad’, or even ‘paranoid’
quickly discontinue it, for obvious reasons.

Rey & Tennant propose two possible reasons for the
well-established link between cannabis use and
psychosis when they ask "Does cannabis cause these
conditions, or do patients use cannabis to relieve their
distress?"

In their paper they mostly cite evidence to support the
former, and to oppose the latter. This is indeed the
pattern emerging from research in the area, but I would
like to point out that a correlation between cannabis
use and psychosis does not necessarily imply
causation in either direction.

I would like to see research investigating other
variables, which may be responsible for the apparent
link. The authors do mention studies that have
controlled for use of other drugs, but a whole host of
social factors including SES (socioeconomic status),
and personal factors including personality, trauma, and
family seem to have been entirely ignored in this
review.

It is not a surprise to hear there is evidence that cannabis usage
has detrimental effects on health, especially mental health. However I
think we should consider cannabis use in the same way that we consider
alcohol use. Substance abuse has a negative impact on health but substance
usage in moderation may not. We know that if one drinks 6 pints of lager
each evening there are many negative effects. However a glass of wine each
evening has been said to be good for you. It is possible that the same may
be true of cannabis. Perhaps there are positive effects of consuming small
amounts of cannabis on occasion and certainly it is likely there are
negative effects from being under it's influence on a regular basis. It
would be interesting to research other effects of cannabis usage and see
if a line can be drawn between usage in moderation and abusive use.